With the development of hand-held electronic devices like smart phone and tablet, such devices become increasingly thinner but with more and more functions, which makes Printed Circuit Board (PCB) design more and more complicated. Thousands of components including packages like CSPs (chip size or chip scale packages), BGAs (ball grid arrays), LGAs (land grid arrays) and POPs (package on packages), and elements like resistors, capacitors and so on are integrated on a small PCB. Generally, CSP/BGA packages are surrounded by lots of elements. Some of such elements are very sensitive, and contamination by underfill adhesives may cause quality issues.
Underfill compositions (also referred as underfill adhesives) are widely used on integrated circuit boards of hand-held electronic devices to improve so called “drop-test” performance and to absorb some of the stress of thermal cycling. Underfill adhesives can flow under CSP/BGA packages with or without board preheating to fill the gap between the package and the PCB by capillary action and form fillet around the package. Then the liquid underfill adhesives become crosslinked solids via heat cure; accordingly provide protection to solder joints.
Generally, underfill adhesives are based on epoxy resins, and additionally contain a latent epoxy curing agent (such as anhydrides, amines, imidazoles, thiols and the like), an accelerator, an active diluent, and other additives (such as wetting agents, adhesion promoters, defoamers and the like). FIG. 1 shows a typical viscosity—temperature profile of a traditional heat curable underfill adhesive. With temperature rising, the viscosity of the underfill adhesive initially reduces and then increases dramatically after the occurrence of curing reaction. Based on this property, a preheating step can be employed to quicken the flow speed of underfill adhesives when their viscosities are too high. However, this property also causes difficulty in controlling the accuracy of the underfill flow during the heat cure. Underfill adhesives may flow out at an elevated temperature and contaminate some resistors and capacitors around the packages, accordingly lead to quality issues.
In order to increase manufacturing throughput and reduce cost, low viscosity underfill adhesives are used widely in the industry. Such low viscosity underfill adhesives flow fast at room temperature, and thus greatly reduce flow time and dispense with the board preheating step, accordingly increase throughput and reduce the process cost. However, the use of such low viscosity underfill adhesives causes a big challenge to accurately control underfill flow. Contamination issue cannot be avoided even with a very accurate control of adhesive volume.
Higher viscosity underfill adhesives have a lower flow speed and can be used to avoid such overflow problems in some degree. However, the manufacturing throughput is reduced therewith.
US2002/128353 and US2004/087681 describe a fluxing no-flow underfill adhesive respectively. Such no-flow underfill adhesive contains an epoxy resin, a hardener and a fluxing agent (an organic acid), and can be cured with a reflow soldering process with or without a post cure. The no-flow property can help to effectively control the flow speed of underfill adhesives. However such no-flow underfill technology has a very narrow reflow operation window to balance soldering and the underfill cure process. It's very difficult to use such no-flow materials in the complicated surface mounting process.
Therefore, there is still a need for underfill compositions which have a low viscosity ensuring a high manufacturing throughput and the flow of which under the packages can be accurately controlled so as not to over flow and not to contaminate the elements surrounding the packages.